Writing instrument



FIG. 1 is a front perspective view of an ornamental design for a writing instrument according to the present invention;

FIG. 2 is a bottom plan view thereof;

FIG. 3 is a top plan view thereof;

FIG. 4 is a back side view thereof;

FIG. 5 is a front side view thereof;

FIG. 6 is a right side view thereof; and,

FIG. 7 is a left side view. 

CLAIM The ornamental design for a writing instrument, as shown and described. 